Active matrix substrate and method of fabricating the same

ABSTRACT

An active matrix substrate includes a substrate composed of resin, and a polysilicon thin film diode formed on the substrate. The polysilicon thin film diode may be a lateral diode centrally having a region into which impurity is doped. As an alternative, the polysilicon thin film diode may be comprised of two lateral diodes electrically connected in parallel to each other and arranged in opposite directions.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to an active matrix substrate partiallyconstituting a liquid crystal display device, and a method offabricating the same.

[0003] 2. Description of the Related Art

[0004]FIG. 1 is a cross-sectional view of a conventional active matrixsubstrate 100 partially constituting a liquid crystal display device.

[0005] The active matrix substrate 100 is comprised of a glass substrate101, a thin chromium (Cr) film 102 formed as a gate electrode partiallyon the glass substrate 101, a silicon nitride film 103 formed as anelectrically insulating film, covering the thin chromium film 102 andthe glass substrate 101 therewith, an amorphous silicon film 104 formedon the silicon nitride film 103, n+ amorphous silicon films 105 formedpartially on the amorphous silicon film 104, a thin chromium (Cr) film106 formed as a barrier film on the n+ amorphous silicon films 105, andan indium tin oxide (ITO) film 107 which will make a pixel electrode andwhich makes contact with the amorphous silicon film 104, the n+amorphous silicon film 105, and the thin chromium film 106, and coversthe silicon nitride film 103 therewith.

[0006] The active matrix substrate 100 is fabricated as follows.

[0007] First, the thin chromium film 102 which will define a gateelectrode is formed on the glass substrate 101 by sputtering. Then, thethin chromium film 102 is patterned into a gate electrode.

[0008] Then; the silicon nitride film 103, the amorphous silicon film104 and the n+ amorphous silicon film 105 are successively formed on theglass substrate 101 by plasma-enhance chemical vapor deposition (PECVD)at 300 degrees centigrade.

[0009] Then, a data wiring layer comprised of the amorphous silicon film104 and the n+ amorphous silicon film 105 is patterned into an island byphotolithography and dry etching.

[0010] Then, the thin chromium film 106 is formed on the n+ amorphoussilicon film 105 by sputtering. The thin chromium film 106 acts as abarrier layer between the data wiring layer and the ITO film 107.

[0011] Then, the thin chromium film 106 and the n+ amorphous siliconfilm 105 are patterned.

[0012] Then, the ITO film 107 which will define a pixel electrode isformed by sputtering, and then, is patterned.

[0013] Thus, the active matrix substrate 100 including a thin filmtransistor having an amorphous silicon film, as a switching device, isfabricated through the above-mentioned steps.

[0014] Since glass has a high specific gravity, the active matrixsubstrate 100 including the glass substrate 101 is relatively heavy.

[0015] In particular, since glass is readily broken, the glass substrate101 has to be formed to have a great thickness, resulting that theactive matrix substrate 100 is unavoidably heavy.

[0016] These days, a liquid crystal display device is required to belight and thin, and hence, an active matrix substrate which is a part ofa liquid crystal display device has to be fabricated lighter andthinner.

[0017] However, for the reasons mentioned above, there is limitation infabricating a liquid crystal display device including a glass substrate,lighter and thinner.

[0018] Consequently, in order to fabricate a liquid crystal displaydevice lighter and thinner, many attempts have been made to use a resinsubstrate in place of a glass substrate, because a resin substrate islighter than a glass substrate and can be fabricated thinner than aglass substrate.

[0019] For instance, Japanese Unexamined Patent Publication No.11-103064 (A) has suggested an active matrix substrate including a thinfilm transistor (TFT) as a switching device which thin film transistoris comprised of a thin polysilicon film formed on a resin substrate.

[0020] A thin film transistor includes a gate insulating film as anindispensable part. A gate insulating film is formed generally byplasma-enhanced chemical vapor deposition (PECVD) or sputtering.

[0021] A resin substrate generally has about 200 degrees centigrade as amaximum resistance to heat. The inventors had conducted variousexperiments, and found out that a gate insulating film formed by PECVDor sputtering at 200 degrees centigrade or lower, which is a maximumresistance of a resin substrate to heat, would have a low density andcause much current leakage, resulting in that the gate insulating filmwas not practicable. Accordingly, even if steps other than a step offorming a gate insulating film were carried out at 200 degreescentigrade or lower, it would be impossible to form a high-quality gateinsulating film.

[0022] In the above-mentioned experiments, the inventors had also foundout that a gate insulating film formed by PECVD or sputtering at 300degrees centigrade or higher had a high density and had caused onlysmall current leakage, and hence, the gate insulating film wassufficiently practicable.

[0023] However, 300 degrees centigrade is over a maximum resistance of aresin substrate to heat. Hence, if PECVD or sputtering were carried outat 300 degrees centigrade or higher for forming a gate insulating film,a resin substrate would be thermally destroyed.

[0024] Japanese Unexamined Patent Publication No. 10-173194 (A) hassuggested a method of fabricating a semiconductor device, including thesteps of forming a first inorganic insulating thin film on a resinsubstrate or resin film without exposing a surface on which the firstinorganic insulating thin film is to be formed, to plasma, forming asecond inorganic insulating thin film on the first inorganic insulatingthin film with the surface being exposed to plasma, and forming a thinsemiconductor film on either the first inorganic insulating thin film orthe second inorganic insulating thin film.

[0025] Japanese Unexamined Patent Publication No. 11-174424 (A) hassuggested a substrate to be used for a liquid crystal display panelwhich substrate is composed of copolymer polycarbonate resin containing3, 3, 5-trimethyl-1,1-di(4-phenol) cyclohexyridene, bisphenol, andbisphenol constituents wherein the bisphenol is contained in the rangeof 30 to 99 mol %.

[0026] Japanese Unexamined Patent Publication No. 7-74374 (A) hassuggested a thin film diode including a first electrode layer formed ona substrate, a semiconductor layer formed on the first electrode layer,a buffer layer formed on the semiconductor layer, and a second electrodelayer formed on the buffer layer, wherein the semiconductor layer andthe buffer layer have almost the same pattern as each other.

[0027] The above-mentioned problem remains unsolved even in theabove-mentioned Publications.

SUMMARY OF THE INVENTION

[0028] In view of the above-mentioned problem in the prior active matrixsubstrates, it is an object of the present invention to provide anactive matrix substrate which includes a resin substrate and is capableof avoiding thermal destruction of a resin substrate.

[0029] In view of the shortcomings in the above-mentioned conventionalactive matrix substrates, the inventors paid attention to a diode whichis not necessary to include a gate insulating film. That is, theinventors selected a diode as a switching device to be used for anactive matrix substrate, in place of a thin film transistor.

[0030] In one aspect of the present invention, there is provided anactive matrix substrate including (a) a substrate composed of resin, and(b) a polysilicon thin film diode formed on the substrate.

[0031] The active matrix substrate in accordance with the presentinvention is not necessary to include a gate insulating film having lowquality and low reliability, unlike a conventional active matrixsubstrate including a thin film transistor, ensuring enhancement inperformances and reliability.

[0032] In addition, it is possible to use a resin substrate having asmaller thickness than a glass substrate in the active matrix substratein accordance with the present invention. Hence, in comparison with anactive matrix substrate including a glass substrate, it would bepossible to reduce a height of an active matrix substrate, and hence, aheight of a liquid crystal display device including the active matrixsubstrate in accordance with the present invention.

[0033] It is preferable that the polysilicon thin film diode is formedas a lateral diode.

[0034] If the polysilicon thin film diode were formed as a verticaldiode, it would be necessary to carry out film deposition and laserannealing a plurality of times. If an upper film is annealed byradiating laser beams thereto, a profile of an impurity concentration ina lower film might be destroyed. Furthermore, if film deposition andlaser annealing were not carried out in vacuum, a natural oxidation filmwould be formed between layers. Since a lateral diode can be formedwithout causing such problems as mentioned above, it is preferable thatthe polysilicon thin film diode is formed as a longitudinal diode.

[0035] It is preferable that the lateral diode centrally has a regioninto which impurity is doped.

[0036] The lateral diode may be designed to have a nin structure, a pipstructure, an ini structure or an ipi structure.

[0037] As an alternative, the lateral diode may be designed to have ni-or pi-Schottky structure.

[0038] The polysilicon thin film diode may be comprised of two lateraldiodes electrically connected in parallel to each other and arranged inopposite directions.

[0039] The substrate may be composed of polyethersulfon, polyimide,polycarbonate or siloxane.

[0040] The active matrix substrate may be designed to further include alight-impermeable film formed below the polysilicon thin film diode.

[0041] The light-impermeable film may be comprised of a chromium film.

[0042] The active matrix substrate in accordance with the presentinvention may be applied to a light-transmission type liquid crystaldisplay device, a COT type liquid crystal display device or alight-reflection type liquid crystal display device.

[0043] In another aspect of the present invention, there is provided amethod of fabricating an active matrix substrate, including the steps of(a) forming an amorphous silicon film on a substrate composed of resin,(b) doping impurity into the amorphous silicon film in a selected regionthereof, (c) radiating laser beams to the amorphous silicon film forcrystallizing the amorphous silicon film into a polysilicon film, and(d) patterning the polysilicon film into an island to thereby form aparallel-type diode.

[0044] There is further provided a method of fabricating an activematrix substrate, including the steps of (a) forming an electricallyinsulating film on a substrate composed of resin, (b) forming anamorphous silicon film on the electrically insulating film, (c) dopingimpurity into the amorphous silicon film in a selected region thereof,(d) radiating laser beams to the amorphous silicon film forcrystallizing the amorphous silicon film into a polysilicon film, (e)patterning the polysilicon film into an island, (f) forming a metalwiring such that the metal wire makes electrical contact with theisland-shaped polysilicon film, (g) forming an interlayer insulatingfilm all over a product resulted from the step (f), (h) forming acontact hole through the interlayer insulating film such that thecontact hole reaches the metal wire, and (i) forming a metal film whichwill define a pixel electrode such that the contact hole is filled withthe metal film.

[0045] The metal film to be formed in the step (i) may be anelectrically conductive transparent film such as an indium tin oxide(ITO) film. The metal film may be annealed.

[0046] The method may further include the step of (j) annealing thepolysilicon film. The step (j) is to be carried out between the steps(d) and (e).

[0047] The method may further include the step of (k) applying hydrogenplasma to the polysilicon film.

[0048] The method may further include the step of (l) forming alight-impermeable film on the resin substrate.

[0049] An active matrix substrate formed by the above-mentioned methodsmay be applied to a light-transmission type liquid crystal displaydevice or a COT type liquid crystal display device.

[0050] There is still further provided a method of fabricating an activematrix substrate, including the steps of (a) forming an electricallyinsulating film on a substrate composed of resin, (b) forming anamorphous silicon film on the electrically insulating film, (c) dopingimpurity into the amorphous silicon film in a selected region thereof,(d) radiating laser beams to the amorphous silicon film forcrystallizing the amorphous silicon film into a polysilicon film, (e)patterning the polysilicon film into an island, (f) forming a metalwiring such that the metal wire makes electrical contact with theisland-shaped polysilicon film, (g) coating a photosensitive film over aproduct resulted from the step (f), exposing the photosensitive film toa light, and developing the photosensitive film to thereby form basesteps in a region in which a pixel is to be formed, (h) forming aninterlayer insulating film all over a product resulted from the step(g), (i) forming a contact hole through the interlayer insulating filmsuch that the contact hole reaches the metal wire, and (j) forming ametal film which will define a pixel electrode such that the contacthole is filled with the metal film.

[0051] The method may further include the step of (k) annealing the basesteps for smoothing the base steps, the step (k) being to be carried outbetween the steps (g) and (h).

[0052] The interlayer insulating film may be formed of the same materialas the material of which the base steps are formed, in the step (h).

[0053] The method may further include the step of annealing the metalfilm.

[0054] An active matrix substrate formed by the above-mentioned methodsmay be applied to a light-reflection type liquid crystal display device.

[0055] The advantages obtained by the aforementioned present inventionwill be described hereinbelow.

[0056] The active matrix substrate in accordance with the presentinvention is no longer necessary to include a gate insulating filmhaving low quality and low reliability. Hence, the active matrixsubstrate in accordance with the present invention could presentenhanced reliability in comparison with a conventional active matrixsubstrate including a thin film transistor.

[0057] In the active matrix substrate and the method of fabricating thesame both in accordance with the present invention, there is not used athin film such as an amorphous silicon film to be formed by PECVD whichthin film is necessary, when formed, to produce a process temperatureequal to or higher than a maximum resistance of a resin substrate toheat. Accordingly, the active matrix substrate and the method offabricating the same both in accordance with the present invention makeit possible to use a resin substrate in place of a glass substrate. Theactive matrix substrate including a resin substrate can be formedlighter and thinner than an active matrix substrate including a glasssubstrate. This ensures that a liquid crystal display device includingthe active matrix substrate can be formed lighter and thinner than aliquid crystal display device including a conventional active matrixsubstrate having a glass substrate.

[0058] The method of fabricating an active matrix substrate inaccordance with the present invention makes it possible to reduce thenumber of photoresist steps in which photolithography and etching arecarried out through the use of a photoresist film, in comparison with aconventional method of fabricating an active matrix substrate includinga thin film transistor. Specifically, a conventional method offabricating an active matrix substrate including a thin film transistorwas necessary to carry out photoresist steps six or seven times. Incontrast, the method of fabricating an active matrix substrate inaccordance with the present invention carries out photoresist steps onlyfive times.

[0059] In addition, the active matrix substrate in accordance with thepresent invention includes a resin substrate thinner than a glasssubstrate. Accordingly, it would be possible to reduce a height of theactive matrix substrate in accordance with the present invention incomparison with an active matrix substrate including a glass substrate.Hence, it would be possible to reduce a height of a liquid crystaldisplay device including the active matrix substrate in accordance withthe present invention in comparison with a liquid crystal display deviceincluding a conventional active matrix substrate including a glasssubstrate.

[0060] The above and other objects and advantageous features of thepresent invention will be made apparent from the following descriptionmade with reference to the accompanying drawings, in which likereference characters designate the same or similar parts throughout thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0061]FIG. 1 is a cross-sectional view of a conventional active matrixsubstrate.

[0062]FIG. 2 is a cross-sectional view of an active matrix substrate inaccordance with the first embodiment of the present invention.

[0063]FIG. 3 is a cross-sectional view of a light-transmission typeliquid crystal display device including the active matrix substrate inaccordance with the first embodiment.

[0064]FIG. 4 is a plan view of the active matrix substrate in thelight-transmission type liquid crystal display device illustrated inFIG. 3, when upwardly viewed.

[0065]FIG. 5 is a plan view of an opposite substrate in thelight-transmission type liquid crystal display device illustrated inFIG. 3, when upwardly viewed.

[0066]FIGS. 6A to 6G are cross-sectional views of the active matrixsubstrate in accordance with the first embodiment, illustratingrespective steps of a method of fabricating the same.

[0067]FIG. 7 is a cross-sectional view of an active matrix substrate inaccordance with the second embodiment of the present invention.

[0068]FIG. 8 is a plan view of a diode mounted on the active matrixsubstrate in accordance with the second embodiment, when upwardlyviewed.

[0069]FIGS. 9A to 9G are cross-sectional views of the active matrixsubstrate in accordance with the second embodiment, illustratingrespective steps of a method of fabricating the same.

[0070]FIG. 10 is a cross-sectional view of an active matrix substrate inaccordance with the third embodiment of the present invention.

[0071]FIGS. 11A to 11G are cross-sectional views of the active matrixsubstrate in accordance with the third embodiment, illustratingrespective steps of a method of fabricating the same.

[0072]FIG. 12 is a cross-sectional view of an active matrix substrate inaccordance with the fourth embodiment of the present invention.

[0073]FIG. 13 is a cross-sectional view of an active matrix substrate inaccordance with the fifth embodiment of the present invention.

[0074]FIG. 14 is a cross-sectional view of a light-reflection typeliquid crystal display device including the active matrix substrate inaccordance with the fifth embodiment.

[0075]FIGS. 15A to 15E are cross-sectional views of the active matrixsubstrate in accordance with the fifth embodiment, illustratingrespective steps of a method of fabricating the same.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0076] Preferred embodiments in accordance with the present inventionwill be explained hereinbelow with reference to drawings.

[0077] [First Embodiment]

[0078]FIG. 2 is a cross-sectional view of an active matrix substrate 10in accordance with the first embodiment of the present invention.

[0079] The active matrix substrate 10 in accordance with the firstembodiment is comprised of a substrate 1 composed of resin, a silicondioxide film 2 formed as an electrically insulating film on the resinsubstrate 1, a diode 11 formed on the silicon dioxide film 2, a chromium(Cr) film 7 formed as a metal wiring film on the silicon dioxide film 2such that the chromium film 7 makes electrical contact with the diode 11at its opposite ends, an interlayer insulating film 8 covering thechromium film 7, the diode 11 and the silicon dioxide film 2 therewith,and an indium tin oxide (ITO) film 9 formed as a pixel electrode on theinterlayer insulating film 8 and filling therewith a contact hole 8 aformed through the interlayer insulating film 8 such that the contacthole 8 a reaches the chromium film 7.

[0080] The resin substrate 1 is composed of polyethersulfon (PES).

[0081] The diode 11 is a lateral diode composed of polysilicon, and hasnin or pip structure. It is preferable that the diode 11 has nin or pipstructure, because a current-voltage (I-V) characteristic has to besymmetrical in driving a liquid crystal display device.

[0082] In the specification, the term “resin substrate” indicates allforms to which a diode can be formed, as well as a plate-shapedsubstrate. For instance, a resin film is covered by the term “resinsubstrate”.

[0083]FIG. 3 is a cross-sectional view of a light-transmission typeliquid crystal display device 20 including the active matrix substrate10 in accordance with the first embodiment.

[0084] The light-transmission type liquid crystal display device 20 iscomprised of the active matrix substrate 10, an opposite substrate 21arranged in facing relation with the active matrix substrate 10, and aliquid crystal layer 23 sandwiched between the active matrix substrate10 and the opposite substrate 21.

[0085] The active matrix substrate 10 further includes an alignment film24 facing the liquid crystal layer 23, and a polarizing plate 25 formedon a bottom surface of the resin substrate 1.

[0086] The opposite substrate 21 is comprised of an electricallyinsulating transparent substrate 26, a black matrix layer 27 formed onthe electrically insulating transparent substrate 26 as alight-impermeable layer, a color layer 28 formed on the electricallyinsulating transparent substrate 26, partially overlapping the blackmatrix layer 27, a transparent overcoat layer 29 covering the blackmatrix layer 27 and the color layer 28 therewith, an alignment film 30formed on the overcoat layer 29, an electrically conductive layer 31formed on a bottom surface of the electrically insulating transparentsubstrate 26, and a polarizing plate 32 covering the electricallyconductive layer 31 therewith.

[0087] The electrically conductive layer 31 prevents electric chargescaused by contact of a liquid crystal display panel with something, fromelectrically influencing the liquid crystal layer 23.

[0088] The color layer 28 is comprised of a resin film containing red(R), green (G) and blue (B) pigments therein.

[0089] The alignment films 24 and 30 are adhered to the active matrixsubstrate 10 and the opposite substrate 21 such that the alignment films24 and 30 face each other, after rubbed such that the liquid crystallayer 23 is homogeneously aligned in a direction inclining by about 10to about 30 degrees from a direction in which a pixel electrode extends.

[0090] In order to ensure a gap between the active matrix substrate 10and the opposite substrate 21, spacers (not illustrated) are sandwichedtherebetween, and the liquid crystal layer 23 is sealed at its peripheryin order to prevent liquid crystal molecules from leaking out of theliquid crystal layer 23.

[0091]FIG. 4 is a plan view of the active matrix substrate 10 viewedfrom the liquid crystal layer 23. FIG. 2 is a cross-sectional view takenalong the line II-II in FIG. 4.

[0092] As illustrated in FIG. 4, the diodes 11 are arranged in matrix onthe resin substrate 1. The chromium film 7 as a pixel electrode isformed in association with each of the diodes 11. The diodes 11 arrangedin a column are electrically connected to one another through a scanningline 12 comprised of the chromium film 7 extending in a direction inwhich the column extends.

[0093]FIG. 5 is a plan view of the opposite substrate 21 viewed from theliquid crystal layer 23, that is, viewed in a direction indicated by anarrow B in FIG. 3.

[0094] As illustrated in FIG. 5, the opposite substrate 21 includes aplurality of signal lines 33 formed thereon extending in parallel to oneanother in a direction perpendicular to a direction in which thescanning line 12 extends.

[0095] The light-transmission type liquid crystal display device 20 maybe driven in any way. For instance, the light-transmission type liquidcrystal display device 20 may be driven in accordance with aconventional process. For instance, one of methods of driving such alight-transmission type liquid crystal display device is disclosed in S.Matsumoto, “Liquid Crystal Display Technique—Active Matrix LCD—”, 1996,pp. 155-158.

[0096] Though the active matrix substrate 10 in accordance with thefirst embodiment is applied to the light-transmission type liquidcrystal display device 20 in FIG. 3, the active matrix substrate 10 maybe applied to a COT type liquid crystal display device.

[0097] Herein, a COT type liquid crystal display device indicates aliquid crystal display device in which a color filter, which correspondsto the color layer 28 illustrated in FIG. 3, is formed on a switchingdevice. Herein, a switching device includes a thin film transistor and adiode. That is, “COT” means both “Color Filter on TFT (Thin FilmTransistor)” and “Color Filter on TFD (Thin Film Diode)”.

[0098]FIGS. 6A to 6G are cross-sectional views of the active matrixsubstrate 10 in accordance with the first embodiment, illustratingrespective steps of a method of fabricating the same. Hereinbelow isexplained a method of fabricating the active matrix substrate 10, withreference to FIGS. 6A to 6G.

[0099] As will be explained in each of the later mentioned steps, atemperature at which each of the steps is carried out is equal to orlower than a maximum resistance of the resin substrate 1 to heat.

[0100] First, as illustrated in FIG. 6A, the silicon dioxide film 2 isformed as a cover film by sputtering on the resin substrate 1 composedof polyethersulfon (PES) having a maximum resistance to heat of about180 degrees centigrade. The silicon dioxide film 2 has a thickness of6000 angstroms.

[0101] Then, the amorphous silicon (a-Si) film 3 is formed by sputteringon the resin substrate 1 so that the amorphous silicon film 3 has athickness of 500 angstroms.

[0102] The conditions for forming the resin substrate 1 and theamorphous silicon film 3 by sputtering are as follows.

[0103] Radio frequency power: 4 kW

[0104] Pressure of argon gas: 5 mtorr

[0105] Temperature of the resin substrate 1: 150 degrees centigrade

[0106] Then as illustrated in FIG. 6B, photoresist is coated all overthe amorphous silicon film 3, and then, the photoresist is patterned byphotolithography and etching to thereby form a mask 4.

[0107] Then, phosphorus (P) is doped into the amorphous silicon film 3through the mask 4 by ion-doping technique. As a result, impurity-dopedregions 5 into which n-type impurity is doped are formed in theamorphous silicon film 3 in selected regions.

[0108] The conditions for carrying out ion-doping are as follows.

[0109] Acceleration voltage: 20 KeV

[0110] Doped phosphorus: 2=10¹⁵ cm⁻²

[0111] After removal of the mask 4, the amorphous silicon film 3 iscrystallized into a polysilicon film 6 by excimer laser annealing, asillustrated in FIG. 6C. The impurity-doped regions 5 are simultaneouslyreformed into the polysilicon film 6 by the excimer laser annealing.

[0112] The conditions for carrying out the excimer laser annealing areas follows.

[0113] Light source: XeCl

[0114] Energy density: 350 mJ/cm²

[0115] Beam diameter: 250×0.4 mm

[0116] Pitch of scanning radiation: 0.04 mm

[0117] Then, the polysilicon film 6 is annealed for an hour at 150degrees centigrade in hydrogen atmosphere.

[0118] Then, after photoresist has been coated over the polysilicon film6, the photoresist is patterned by photolithography and dry etching tothereby form a mask. Then, the polysilicon film 6 is patterned by dryetching into an island through the mask, as illustrated in FIG. 6D.

[0119] Then, a chromium film which will make a metal wiring layer 7 isformed by sputtering, entirely covering the island-shaped polysiliconfilm 6 and the silicon dioxide film 2 therewith.

[0120] The conditions for forming the chromium film by sputtering are asfollows.

[0121] Radio frequency power: 4 kW

[0122] Pressure of argon gas: 5 mtorr

[0123] Temperature of the resin substrate 1: 150 degrees centigrade

[0124] Then, after photoresist has been coated on the chromium film, thephotoresist is patterned by photolithography and dry etching to therebyform a mask. The chromium film is patterned through the thus formed maskto thereby form the metal wiring layer 7 such that the metal wiringlayer 7 partially overlaps the island-shaped polysilicon film 6, asillustrated in FIG. 6E.

[0125] Then, as illustrated in FIG. 6F, a silicon dioxide film whichwill make the interlayer insulating film 8 is formed by sputteringentirely covering the silicon dioxide film 2, the metal wiring layer 7and the polysilicon film 6 therewith.

[0126] The conditions for forming the silicon dioxide film by sputteringare as follows.

[0127] Radio frequency power: 4 kW

[0128] Pressure of argon gas: 5.2 mtorr

[0129] Temperature of the resin substrate 1: 150 degrees centigrade

[0130] Then, after photoresist has been coated on the interlayerinsulating film 8, the photoresist is patterned by photolithography anddry etching to thereby form a mask. Then, the interlayer insulating film8 is formed therethrough with a contact hole 8 a reaching the metalwiring layer 7 through the use of the mask.

[0131] Then, an electrically conductive transparent film such as anindium tin oxide (ITO) film is formed over the interlayer insulatingfilm 8 by sputtering such that the contact hole 8 a is filled with theinterlayer insulating film 8.

[0132] The conditions for forming the electrically conductivetransparent film by sputtering are as follows.

[0133] Radio frequency power: 4 kW

[0134] Pressure of argon gas: 5.5 mtorr

[0135] Temperature of the resin substrate 1: 155 degrees centigrade

[0136] Then, after photoresist has been coated on the electricallyconductive transparent film, the photoresist is patterned byphotolithography and dry etching to thereby form a mask. Then, theelectrically conductive transparent film is patterned through the thusformed mask to thereby form a pixel electrode 9, as illustrated in FIG.6G.

[0137] Then, a product resulted from the above-mentioned steps isannealed for an hour at 150 degrees centigrade in order to reduce acontact resistance.

[0138] Through the above-mentioned steps, a polysilicon lateral diodehaving nin structure has been formed on the resin substrate 1. Sincecurrent-voltage (I-V) characteristic has to be symmetrical in driving aliquid crystal display device, it is preferable that the diode has ninstructure.

[0139] As having been explained so far, the active matrix substrate 10in accordance with the first embodiment is no longer necessary toinclude a gate insulating film having low quality and low reliability.Hence, the active matrix substrate 10 in accordance with the firstembodiment could present enhanced performances and reliability incomparison with a conventional active matrix substrate including a thinfilm transistor.

[0140] In the active matrix substrate 10 in accordance with the firstembodiment, there is not used a thin film such as an amorphous siliconfilm to be formed by PECVD which thin film is necessary, when formed, toproduce a process temperature equal to or higher than a maximumresistance of the resin substrate 1 to heat (specifically, 180 degreescentigrade). Accordingly, the active matrix substrate 10 in accordancewith the first embodiment make it possible to use the resin substrate 1in place of a glass substrate. The active matrix substrate 10 includingthe resin substrate 1 can be formed lighter and thinner than aconventional active matrix substrate including a glass substrate. Thisensures that a liquid crystal display device including the active matrixsubstrate 10 can be formed lighter and thinner than a liquid crystaldisplay device including a conventional active matrix substrate having aglass substrate.

[0141] The method of fabricating the active matrix substrate 10 inaccordance with the first embodiment makes it possible to reduce thenumber of photoresist steps in which photolithography and etching arecarried out through the use of a patterned photoresist film, incomparison with a conventional method of fabricating an active matrixsubstrate including a thin film transistor. Specifically, a conventionalmethod of fabricating an active matrix substrate including a thin filmtransistor was necessary to carry out photoresist steps six or seventimes. In contrast, the method of fabricating the active matrixsubstrate 10 in accordance with the first embodiment carries outphotoresist steps only five times.

[0142] In addition, the active matrix substrate 10 in accordance withthe first embodiment includes the resin substrate 1 thinner than a glasssubstrate. Accordingly, it would be possible to reduce a height of theactive matrix substrate 10 in accordance with the first embodiment incomparison with an active matrix substrate including a glass substrate.Hence, it would be possible to reduce a height of a liquid crystaldisplay device including the active matrix substrate 10 in accordancewith the first embodiment in comparison with a liquid crystal displaydevice including a conventional active matrix substrate including aglass substrate.

[0143] [Second Embodiment]

[0144]FIG. 7 is a cross-sectional view of an active matrix substrate 40in accordance with the second embodiment of the present invention.

[0145] The active matrix substrate 40 in accordance with the secondembodiment is comprised of a substrate 41 composed of resin, a silicondioxide film 42 formed as an electrically insulating film on the resinsubstrate 41, a diode 43 formed on the silicon dioxide film 42, achromium (Cr) film 47 formed as a metal wiring film on the silicondioxide film 42 such that the chromium film 47 makes electrical contactwith the diode 43 at its opposite ends, an interlayer insulating film 48covering the chromium film 47, the diode 43 and the silicon dioxide film42 therewith, and an indium tin oxide (ITO) film 49 formed as a pixelelectrode on the interlayer insulating film 48 and filling therewith acontact hole 48 a formed through the interlayer insulating film 48 suchthat the contact hole 48 a reaches the chromium film 47.

[0146] The resin substrate 41 is composed of polyimide (PI).

[0147] The diode 43 is a lateral diode composed of polysilicon, and hasni or pi structure. When a liquid crystal display device is driven bymeans of a diode having asymmetrical structure such as the diode 43, twodiodes are electrically connected in ring to each other in order toensure symmetry in current-voltage (I-V) characteristic.

[0148]FIG. 8 illustrates an example of ring connection of diodes. InFIG. 8, two ni Schottky type polysilicon lateral diodes are electricallyconnected to each other in ring connection.

[0149] As illustrated in FIG. 8, two diodes are electrically connectedin parallel and in opposite directions to each other in the ringconnection of diodes. Specifically, in ring connection of diodes, afirst diode 51 and a second diode 52 are arranged in such a way that animpurity-doped region 51 a of the first diode 51 faces a polysiliconregion 52 b of the second diode 52, and a polysilicon region 51 b of thefirst diode 51 faces an impurity-doped region 52 a of the second diode52.

[0150] Similarly to the active matrix substrate 10 in accordance withthe first embodiment, the active matrix substrate 40 in accordance withthe second embodiment may be applied to a light-transmission type liquidcrystal display device illustrated in FIG. 3. The active matrixsubstrate 40 in accordance with the second embodiment may be applied toa COT type liquid crystal display device.

[0151]FIGS. 9A to 9G are cross-sectional views of the active matrixsubstrate 40 in accordance with the second embodiment, illustratingrespective steps of a method of fabricating the same. Hereinbelow isexplained a method of fabricating the active matrix substrate 40, withreference to FIGS. 9A to 9G.

[0152] As will be explained in each of the later mentioned steps, atemperature at which each of the steps is carried out is equal to orlower than a maximum resistance of the resin substrate 41 to heat.

[0153] First, as illustrated in FIG. 9A, the silicon dioxide film 42 isformed as a cover film by sputtering on the resin substrate 41 composedof polyimide (PI) having a maximum resistance to heat of about 220degrees centigrade. The silicon dioxide film 42 has a thickness of 6000angstroms. 1.9

[0154] Then, the amorphous silicon (a-Si) film 43 is formed bysputtering on the resin substrate 41 so that the amorphous silicon film43 has a thickness of 500 angstroms.

[0155] The conditions for forming the resin substrate 41 and theamorphous silicon film 43 by sputtering are as follows.

[0156] Radio frequency power: 4 kW

[0157] Pressure of argon gas: 5 mtorr

[0158] Temperature of the resin substrate 41: 150 degrees centigrade

[0159] Then as illustrated in FIG. 9B, photoresist is coated all overthe amorphous silicon film 43, and then, the photoresist is patterned byphotolithography and etching to thereby form a mask 44.

[0160] Then, phosphorus (P) is doped into the amorphous silicon film 43through the mask 44 by ion-doping technique. As a result, animpurity-doped region 45 into which n-type impurity is doped is formedin the amorphous silicon film 43 in a selected region.

[0161] The conditions for carrying out ion-doping are as follows.

[0162] Acceleration voltage: 20 KeV

[0163] Doped phosphorus: 2×10¹⁵ cm⁻²

[0164] After removal of the mask 44, the amorphous silicon film 43 iscrystallized into a polysilicon film 46 by excimer laser annealing, asillustrated in FIG. 9C. The impurity-doped region 45 is simultaneouslyreformed into the polysilicon film 46 by the excimer laser annealing.

[0165] The conditions for carrying out the excimer laser annealing areas follows.

[0166] Light source: XeCl

[0167] Energy density: 350 mJ/cm²

[0168] Beam diameter: 250×0.4 mm

[0169] Pitch of scanning radiation: 0.04 mm

[0170] Then, hydrogen plasma is applied to the polysilicon film 46.

[0171] The conditions for applying hydrogen plasma to the polysiliconfilm 46 are as follows.

[0172] Discharge power: 300 W

[0173] Pressure of hydrogen gas: 1 torr

[0174] Temperature of the resin substrate 41: 200 degrees centigrade

[0175] Then, after photoresist has been coated over the polysilicon film46, the photoresist is patterned by photolithography and dry etching tothereby form a mask. Then, the polysilicon film 46 is patterned by dryetching into an island through the mask, as illustrated in FIG. 9D.

[0176] Then, a chromium film which will make the metal wiring layer 47is formed by sputtering, entirely covering the island-shaped polysiliconfilm 46 and the silicon dioxide film 42 therewith.

[0177] The conditions for forming the chromium film by sputtering are asfollows.

[0178] Radio frequency power: 4 kW

[0179] Pressure of argon gas: 5 mtorr

[0180] Temperature of the resin substrate 41: 150 degrees centigrade

[0181] Then, after photoresist has been coated on the chromium film, thephotoresist is patterned by photolithography and dry etching to therebyform a mask. The chromium film is patterned through the thus formed maskto thereby form the metal wiring layer 47 such that the metal wiringlayer 47 partially overlaps the island-shaped polysilicon film 46, asillustrated in FIG. 9E.

[0182] Then, as illustrated in FIG. 9F, a silicon dioxide film whichwill make the interlayer insulating film 48 is formed by sputteringentirely covering the silicon dioxide film 42, the metal wiring layer 47and the polysilicon film 46 therewith.

[0183] The conditions for forming the silicon dioxide film by sputteringare as follows.

[0184] Radio frequency power: 4 kW

[0185] Pressure of argon gas: 5 mtorr

[0186] Temperature of the resin substrate 41: 150 degrees centigrade

[0187] Then, after photoresist has been coated on the interlayerinsulating film 48, the photoresist is patterned by photolithography anddry etching to thereby form a mask. Then, the interlayer insulating film48 is formed therethrough with a contact hole 48 a reaching the metalwiring layer 47, through the use of the mask.

[0188] Then, an electrically conductive transparent film such as anindium tin oxide (ITO) film is formed over the interlayer insulatingfilm 48 by sputtering such that the contact hole 48 a is filled with theinterlayer insulating film 48.

[0189] The conditions for forming the electrically conductivetransparent film by sputtering are as follows.

[0190] Radio frequency power: 4 kW

[0191] Pressure of argon gas: 5 mtorr

[0192] Temperature of the resin substrate 41: 150 degrees centigrade

[0193] Then, after photoresist has been coated on the electricallyconductive transparent film, the photoresist is patterned byphotolithography and dry etching to thereby form a mask. Then, theelectrically conductive transparent film is patterned through the thusformed mask to thereby form a pixel electrode 9, as illustrated in FIG.9G.

[0194] Then, a product resulted from the above-mentioned steps isannealed for an hour at 200 degrees centigrade in order to reduce acontact resistance.

[0195] Through the above-mentioned steps, a polysilicon lateral diodehaving ni structure has been formed on the resin substrate 41.

[0196] The active matrix substrate 40 in accordance with the secondembodiment provides the same advantages as the advantages obtained bythe active matrix substrate 10 in accordance with the first embodiment.

[0197] [Third Embodiment]

[0198]FIG. 10 is a cross-sectional view of an active matrix substrate 60in accordance with the third embodiment of the present invention.

[0199] The active matrix substrate 60 in accordance with the thirdembodiment is comprised of a substrate 61 composed of resin, a silicondioxide film 62 formed as an electrically insulating film on the resinsubstrate 61, a diode 63 formed on the silicon dioxide film 62, achromium (Cr) film 67 formed as a metal wiring film on the silicondioxide film 62 such that the chromium film 67 makes electrical contactwith the diode 63 at its opposite ends, an interlayer insulating film 68covering the chromium film 67, the diode 63 and the silicon dioxide film62 therewith, and an indium tin oxide (ITO) film 69 formed as a pixelelectrode on the interlayer insulating film 68 and filling therewith acontact hole 68 a formed through the interlayer insulating film 68 suchthat the contact hole 68 a reaches the chromium film 67.

[0200] The resin substrate 61 is composed of polycarbonate (PC).

[0201] The diode 63 is a lateral diode composed of polysilicon, and hasini or ipi structure. Ini or ipi structure in a lateral diodecorresponds to a so-called back-to-back structure in which ni or piSchottky structures in a vertical diode are electrically connected toeach other in opposite directions, and has high symmetry in I-Vcharacteristic.

[0202] Similarly to the active matrix substrate 10 in accordance withthe first embodiment, the active matrix substrate 60 in accordance withthe third embodiment may be applied to a light-transmission type liquidcrystal display device illustrated in FIG. 3. The active matrixsubstrate 60 in accordance with the third embodiment may be applied to aCOT type liquid crystal display device.

[0203]FIGS. 11A to 11G are cross-sectional views of the active matrixsubstrate 60 in accordance with the third embodiment, illustratingrespective steps of a method of fabricating the same. Hereinbelow isexplained a method of fabricating the active matrix substrate 60, withreference to FIGS. 11A to 11G.

[0204] As will be explained in each of the later mentioned steps, atemperature at which each of the steps is carried out is equal to orlower than a maximum resistance of the resin substrate 61 to heat.

[0205] First, as illustrated in FIG. 11A, the silicon dioxide film 62 isformed as a cover film by sputtering on the resin substrate 61 composedof polycarbonate (PC) having a maximum resistance to heat of about 130degrees centigrade. The silicon dioxide film 62 has a thickness of 6000angstroms.

[0206] Then, the amorphous silicon (a-Si) film 63 is formed bysputtering on the resin substrate 61 so that the amorphous silicon film63 has a thickness of 500 angstroms.

[0207] The conditions for forming the resin substrate 61 and theamorphous silicon film 63 by sputtering are as follows.

[0208] Radio frequency power: 4 kW

[0209] Pressure of argon gas: 5 mtorr

[0210] Temperature of the resin substrate 61: 150 degrees centigrade

[0211] Then as illustrated in FIG. 11B, photoresist is coated all overthe amorphous silicon film 63, and then, the photoresist is patterned byphotolithography and etching to thereby form a mask 64.

[0212] Then, phosphorus (P) is doped into the amorphous silicon film 63through the mask 64 by ion-doping technique. As a result, animpurity-doped region 65 into which n-type impurity is doped is formedin the amorphous silicon film 63 in a selected region.

[0213] The conditions for carrying out ion-doping are as follows.

[0214] Acceleration voltage: 20 KeV

[0215] Doped phosphorus: 2×10¹⁵ cm⁻²

[0216] After removal of the mask 64, the amorphous silicon film 63 iscrystallized into a polysilicon film 66 by excimer laser annealing, asillustrated in FIG. 11C. The impurity-doped region 65 is simultaneouslyreformed into the polysilicon film 66 by the excimer laser annealing.

[0217] The conditions for carrying out the excimer laser annealing areas follows.

[0218] Light source: XeCl

[0219] Energy density: 350 mJ/cm²

[0220] Beam diameter: 250×0.4 mm

[0221] Pitch of scanning radiation: 0.04 mm

[0222] Then, hydrogen plasma is applied to the polysilicon film 66.

[0223] The conditions for applying hydrogen plasma to the polysiliconfilm 66 are as follows.

[0224] Discharge power: 300 W

[0225] Pressure of hydrogen gas: 1 torr

[0226] Temperature of the resin substrate 61: 100 degrees centigrade

[0227] Then, after photoresist has been coated over the polysilicon film66, the photoresist is patterned by photolithography and dry etching tothereby form a mask. Then, the polysilicon film 66 is patterned by dryetching into an island through the mask, as illustrated in FIG. 11D.

[0228] Then, a chromium film which will make the metal wiring layer 67is formed by sputtering, entirely covering the island-shaped polysiliconfilm 66 and the silicon dioxide film 62 therewith.

[0229] The conditions for forming the chromium film by sputtering are asfollows.

[0230] Radio frequency power: 4 kW

[0231] Pressure of argon gas: 5 mtorr

[0232] Temperature of the resin substrate 61: 150 degrees centigrade

[0233] Then, after photoresist has been coated on the chromium film, thephotoresist is patterned by photolithography and dry etching to therebyform a mask. The chromium film is patterned through the thus formed maskto thereby form the metal wiring layer 67 such that the metal wiringlayer 67 partially overlaps the island-shaped polysilicon film 66, asillustrated in FIG. 11E.

[0234] Then, as illustrated in FIG. 1F, a silicon dioxide film whichwill make the interlayer insulating film 68 is formed by sputteringentirely covering the silicon dioxide film 62, the metal wiring layer 67and the polysilicon film 66 therewith.

[0235] The conditions for forming the silicon dioxide film by sputteringare as follows.

[0236] Radio frequency power: 4 kW

[0237] Pressure of argon gas: 5 mtorr

[0238] Temperature of the resin substrate 61: 150 degrees centigrade

[0239] Then, after photoresist has been coated on the interlayerinsulating film 68, the photoresist is patterned by photolithography anddry etching to thereby form a mask. Then, the interlayer insulating film68 is formed therethrough with a contact hole 68 a reaching the metalwiring layer 67, through the use of the mask.

[0240] Then, an electrically conductive transparent film such as anindium tin oxide (ITO) film is formed over the interlayer insulatingfilm 68 by sputtering such that the contact hole 68 a is filled with theinterlayer insulating film 68.

[0241] The conditions for forming the electrically conductivetransparent film by sputtering are as follows.

[0242] Radio frequency power: 4 kW

[0243] Pressure of argon gas: 5 mtorr

[0244] Temperature of the resin substrate 61: 150 degrees centigrade

[0245] Then, after photoresist has been coated on the electricallyconductive transparent film, the photoresist is patterned byphotolithography and dry etching to thereby form a mask. Then, theelectrically conductive transparent film is patterned through the thusformed mask to thereby form a pixel electrode 69, as illustrated in FIG.11G.

[0246] Then, a product resulted from the above-mentioned steps isannealed for an hour at 130 degrees centigrade in order to reduce acontact resistance.

[0247] Through the above-mentioned steps, a polysilicon lateral diodehaving ini structure has been formed on the resin substrate 61.

[0248] The active matrix substrate 60 in accordance with the thirdembodiment provides the same advantages as the advantages obtained bythe active matrix substrate 10 in accordance with the first embodiment.

[0249] [Fourth Embodiment]

[0250]FIG. 12 is a cross-sectional view of an active matrix substrate 70in accordance with the fourth embodiment of the present invention.

[0251] The active matrix substrate 70 in accordance with the fourthembodiment is comprised of a substrate 61 composed of resin, alight-impermeable film 71 comprised of a chromium film formed on theresin substrate 61, a silicon dioxide film 62 formed as an electricallyinsulating film on the resin substrate 61, covering thelight-impermeable film 71 therewith, a diode 63 formed on the silicondioxide film 62, a chromium (Cr) film 67 formed as a metal wiring filmon the silicon dioxide film 62 such that the chromium film 67 makeselectrical contact with the diode 63 at its opposite ends, an interlayerinsulating film 68 covering the chromium film 67, the diode 63 and thesilicon dioxide film 62 therewith, and an indium tin oxide (ITO) film 69formed as a pixel electrode on the interlayer insulating film 68 andfilling therewith a contact hole 68 a formed through the interlayerinsulating film 68 such that the contact hole 68 a reaches the chromiumfilm 67.

[0252] In comparison with the active matrix substrate 60 in accordancewith the third embodiment, the active matrix substrate 70 in accordancewith the fourth embodiment further includes the light-impermeable film71. The active matrix substrate 70 has the same structure as thestructure of the active matrix substrate 60 except additionally havingthe light-impermeable film 71.

[0253] The active matrix substrate 70 in accordance with the fourthembodiment provides the same advantages as the advantages obtained bythe active matrix substrate 60 in accordance with the third embodiment.In addition, since the active matrix substrate 70 further includes thelight-impermeable film 71, the active matrix substrate 70 would make itpossible to prevent malfunction of a lateral diode having no lowerelectrode, caused by backlight in a light-transmission type liquidcrystal display device.

[0254] Similarly to the active matrix substrate 10 in accordance withthe first embodiment, the active matrix substrate 70 in accordance withthe fourth embodiment may be applied to a light-transmission type liquidcrystal display device illustrated in FIG. 3. The active matrixsubstrate 70 in accordance with the fourth embodiment may be applied toa COT type liquid crystal display device.

[0255] A method of fabricating the active matrix substrate 70 inaccordance with the fourth embodiment has the same steps as the steps tobe carried out in the method of fabricating the active matrix substrate60 in accordance with the third embodiment, except a step of forming thelight-impermeable film 71.

[0256] Specifically, in a method of fabricating the active matrixsubstrate 70 in accordance with the fourth embodiment, a chromium filmwhich will make the light-impermeable film 71 is first formed on theresin substrate 61 by sputtering. The chromium film has a thickness of1500 angstroms.

[0257] Then, after photoresist has been coated on the chromium film, thephotoresist is patterned by photolithography and dry etching to therebyform a mask. Then, the chromium film is patterned through the thusformed mask to thereby form the light-impermeable film 71.

[0258] Then, the steps having been explained with reference to FIGS. 11Bto 11G are carried out.

[0259] Through the above-mentioned steps, a polysilicon lateral diodehaving ini structure has been formed on the resin substrate 61.

[0260] [Fifth Embodiment]

[0261]FIG. 13 is a cross-sectional view of an active matrix substrate 80in accordance with the fifth embodiment of the present invention.

[0262] The active matrix substrate 80 in accordance with the fifthembodiment is comprised of a substrate 81 composed of resin, a silicondioxide film 82 formed as an electrically insulating film on the resinsubstrate 81, a diode 83 formed on the silicon dioxide film 82, basesteps 84 formed on the silicon dioxide film 82 in a region in which apixel is to be formed, a chromium (Cr) film 85 formed as a metal wiringfilm on the silicon dioxide film 82 such that the chromium film 85 makeselectrical contact with the diode 83 at its opposite ends, an interlayerinsulating film 86 covering the chromium film 85, the diode 83, thesilicon dioxide film 82 and the base steps 84 therewith, and an indiumtin oxide (ITO) film 87 formed as a pixel electrode on the interlayerinsulating film 86 and filling therewith a contact hole 86 a formedthrough the interlayer insulating film 86 such that the contact hole 86a reaches the chromium film 85.

[0263] The resin substrate 81 is composed of siloxane.

[0264] The diode 83 is a lateral diode composed of polysilicon, and hasini or ipi structure.

[0265] Since a resin substrate generally has greater optical anisotropythan that of a glass substrate, it is preferable for a resin substrateto be used in a light-reflection type liquid crystal display deviceincluding only one substrate in optical path, with respect to displayquality.

[0266] In addition, since light is not directly radiated to the diode 83in a light-reflection type liquid crystal display device, it is notnecessary for the active matrix substrate 80 to include thelight-impermeable film 71 illustrated in FIG. 12.

[0267]FIG. 14 is a cross-sectional view of a light-reflection typeliquid crystal display device 90 including the active matrix substrate80 in accordance with the fifth embodiment.

[0268] The light-reflection type liquid crystal display device 90 iscomprised of the active matrix substrate 80, an opposite substrate 91arranged in facing relation with the active matrix substrate 80, and aliquid crystal layer 92 sandwiched between the active matrix substrate80 and the opposite substrate 91.

[0269] The active matrix substrate 80 further includes an alignment film93 facing the liquid crystal layer 92.

[0270] The opposite substrate 91 is comprised of an electricallyinsulating transparent substrate 95, a color layer 96 formed on theelectrically insulating transparent substrate 95, a transparent overcoatlayer 97 covering the color layer 96 therewith, an alignment film 98formed on the overcoat layer 97, a phase difference plate 99 formed onthe electrically insulating transparent substrate 95 at the oppositeside of the liquid crystal layer 92, and a polarizing plate 88 formed onthe phase difference plate 99.

[0271]FIGS. 15A to 15E are cross-sectional views of the active matrixsubstrate 80 in accordance with the fifth embodiment, illustratingrespective steps of a method of fabricating the same. Hereinbelow isexplained a method of fabricating the active matrix substrate 80, withreference to FIGS. 15A to 15E.

[0272] As will be explained in each of the later mentioned steps, atemperature at which each of the steps is carried out is equal to orlower than a maximum resistance of the resin substrate 81 to heat.

[0273] First, as illustrated in FIG. 15A, the diode 83 and the metalwiring layer 85 are formed on the resin substrate 81 composed ofsiloxane having a maximum resistance of 250 degrees centigrade to heat,in the same way as the first embodiment.

[0274] Then, as illustrated in FIG. 15B, a photosensitive organic filmis formed on the silicon dioxide film 82. The photosensitive organicfilm is patterned by exposing to light and developing, to thereby thebase steps 84 in a region in which a pixel is to be formed.

[0275] Then, if necessary, the base steps 84 are tightened by baking at100 degrees centigrade.

[0276] Then, as illustrated in FIG. 15C, the base steps 84 are annealedfor an hour at 200 degrees centigrade to thereby smooth or round thebase steps 84.

[0277] Then, as illustrated in FIG. 15D, an organic film which will makethe interlayer insulating film 86 is formed entirely covering the diode83, the metal wiring layer 85, the base steps 84 and the silicon dioxidefilm 82 therewith.

[0278] Then, after photoresist has been coated on the organic film, thephotoresist is patterned by photolithography and dry etching to therebyform a mask. Then, the interlayer insulating film 86 is formedtherethrough with a contact hole 86 a reaching the metal wiring layer85, through the use of the mask.

[0279] Then, an aluminum film 87 is formed over the interlayerinsulating film 86 by sputtering such that the contact hole 86 a isfilled with the aluminum film 87.

[0280] The conditions for forming the aluminum film 87 by sputtering areas follows.

[0281] Radio frequency power: 4 kW

[0282] Pressure of argon gas: 5 mtorr

[0283] Temperature of the resin substrate 61: 170 degrees centigrade

[0284] Then, after photoresist has been coated on the aluminum film 87,the photoresist is patterned by photolithography and dry etching tothereby form a mask. Then, the aluminum film 87 is patterned through thethus formed mask to thereby form a pixel electrode 87, as illustrated inFIG. 15E.

[0285] Then, a product resulted from the above-mentioned steps isannealed for an hour at 150 degrees centigrade in order to reduce acontact resistance.

[0286] Through the above-mentioned steps, a polysilicon lateral diodehaving ini structure has been formed on the resin substrate 81.

[0287] As mentioned earlier, the active matrix substrate 80 inaccordance with the fifth embodiment is suitable to a light-reflectiontype liquid crystal display device.

[0288] The active matrix substrate 80 in accordance with the fifthembodiment provides the same advantages as the advantages obtained bythe active matrix substrate 10 in accordance with the first embodiment.

[0289] In the above-mentioned first to fifth embodiments, only parts bywhich the present invention is characterized have been explained, andparts known to those skilled in the art were not explained in detail.However, it should be noted that even if they are not explained indetail, those skilled in the art could understand them readily.

[0290] While the present invention has been described in connection withcertain preferred embodiments, it is to be understood that the subjectmatter encompassed by way of the present invention is not to be limitedto those specific embodiments. On the contrary, it is intended for thesubject matter of the invention to include all alternatives,modifications and equivalents as can be included within the spirit andscope of the following claims.

[0291] The entire disclosure of Japanese Patent Application No.2001-104570 filed on Apr. 3, 2001 including specification, claims,drawings and summary is incorporated herein by reference in itsentirety.

What is claimed is:
 1. An active matrix substrate comprising: (a) asubstrate composed of resin; and (b) a polysilicon thin film diodeformed on said substrate.
 2. The active matrix substrate as set forth inclaim 1, wherein said polysilicon thin film diode is a lateral diode. 3.The active matrix substrate as set forth in claim 2, wherein saidlateral diode centrally has a region into which impurity is doped. 4.The active matrix substrate as set forth in claim 1, wherein saidpolysilicon thin film diode is comprised of two lateral diodeselectrically connected in parallel to each other and arranged inopposite directions.
 5. The active matrix substrate as set forth inclaim 1, wherein said substrate is composed of polyimide.
 6. The activematrix substrate as set forth in claim 1, wherein said substrate iscomposed of polycarbonate.
 7. The active matrix substrate as set forthin claim 1, wherein said substrate is composed of siloxane.
 8. Theactive matrix substrate as set forth in claim 1, wherein said substrateis composed of polyethersulfon.
 9. The active matrix substrate as setforth in claim 1, further comprising a light-impermeable film formedbelow said polysilicon thin film diode.
 10. A method of fabricating anactive matrix substrate, comprising the steps of: (a) forming anamorphous silicon film on a substrate composed of resin; (b) dopingimpurity into said amorphous silicon film in a selected region thereof;(c) radiating laser beams to said amorphous silicon film forcrystallizing said amorphous silicon film into a polysilicon film; and(d) patterning said polysilicon film into an island to thereby form aparallel-type diode.
 11. A method of fabricating an active matrixsubstrate, comprising the steps of: (a) forming an electricallyinsulating film on a substrate composed of resin; (b) forming anamorphous silicon film on said electrically insulating film; (c) dopingimpurity into said amorphous silicon film in a selected region thereof;(d) radiating laser beams to said amorphous silicon film forcrystallizing said amorphous silicon film into a polysilicon film; (e)patterning said polysilicon film into an island; (f) forming a metalwiring such that said metal wire makes electrical contact with saidisland-shaped polysilicon film; (g) forming an interlayer insulatingfilm all over a product resulted from said step (f); (h) forming acontact hole through said interlayer insulating film such that saidcontact hole reaches said metal wire; and (i) forming a metal film whichwill define a pixel electrode such that said contact hole is filled withsaid metal film.
 12. The method as set forth in claim 11, furthercomprising the step of (j) annealing said polysilicon film, said step(j) being to be carried out between said steps (d) and (e).
 13. Themethod as set forth in claim 11, further comprising the step of (k)applying hydrogen plasma to said polysilicon film.
 14. The method as setforth in claim 11, further comprising the step of (l) forming alight-impermeable film on said substrate.
 15. A method of fabricating anactive matrix substrate, comprising the steps of: (a) forming anelectrically insulating film on a substrate composed of resin; (b)forming an amorphous silicon film on said electrically insulating film;(c) doping impurity into said amorphous silicon film in a selectedregion thereof; (d) radiating laser beams to said amorphous silicon filmfor crystallizing said amorphous silicon film into a polysilicon film;(e) patterning said polysilicon film into an island; (f) forming a metalwiring such that said metal wire makes electrical contact with saidisland-shaped polysilicon film; (g) coating a photosensitive film over aproduct resulted from said step (f), exposing said photosensitive filmto a light, and developing said photosensitive film to thereby form basesteps in a region in which a pixel is to be formed; (h) forming aninterlayer insulating film all over a product resulted from said step(g); (i) forming a contact hole through said interlayer insulating filmsuch that said contact hole reaches said metal wire; and (j) forming ametal film which will define a pixel electrode such that said contacthole is filled with said metal film.
 16. The method as set forth inclaim 15, further comprising the step of (k) annealing said base stepsfor smoothing said base steps, said step (k) being to be carried outbetween said steps (g) and (h).
 17. The method as set forth in claim 15,wherein said interlayer insulating film is formed of the same materialas the material of which said base steps are formed, in said step (h).